The invention relates to total joint replacement devices having improved fracture toughness, and to a method of making them. The devices can be used in total joint replacements such as hip, knee, elbow and shoulder.
Ultra High Molecular Weight Polyethylene (hereafter referred to as UHMWPE or simply xe2x80x9cpolyethylenexe2x80x9d) has been the main material of choice as the-bearing surface in total joint replacement devices. This includes predominately hip, knee, replacements but has been used in shoulder, elbow, ankle and mandibular joint replacements. The success rate of implantation of hip and knee devices is generally quite high but the life of these devices is often limited by the wear and damage of the polyethylene components.
UHMWPE is commercially produced as a powder and is available in several grades from several companies. The grades generally differ in molecular weight and molecular weight distribution. The powder is fabricated into devices by one of three methods: (1) extrusion into bars followed by machining of the device and (2) compression molding into sheets followed by machining and (3) direct compression molding. Each method has advantages and disadvantages.
Extrusion involves introducing a fixed amount polyethylene powder into a chamber; pushing this powder into a heated cylindrical barrel with a ram which then retracts, leaving the chamber empty and waits for the next fixed amount of powder. The process is continuous and each push of the ram advances the polyethylene through the heated barrel. In this manner, the powder is consolidated into a continuous bar with typically a round cross-section. The resulting bar stock is generally from one to six inches in diameter. Implants are then machined from this cylindrical bar stock.
Compression Sheet Molding involves introducing powder into a container that can be as large as 8 inches deep, 4 feet wide and 8 feet long. A platen large enough to cover the entire container is then used to apply heat and pressure to the polyethylene in the container. Implants are then machined from this sheet.
Direct Molding is different from sheet molding in that powder is placed into a mold that compresses it into the final shape of the device without need for machining (rather than a bar or a sheet which needs to be machined into the device shape). (Some machining may be used as part of finishing operations.) The mold containing the powder is heated under pressure to consolidate the polyethylene and form the device. Devices formed in this fashion often exhibit a highly glossy surface finish. The process conditions of direct molding are often quite different from compression molding sheet and different properties are often obtained.
In total joint replacement devices, polyethylene wear and damage takes one or more of the following forms (modes): burnishing (polishing); abrasion (generation of small particles); pitting (formation of pits); delamination (loss of xe2x80x98sheetsxe2x80x99 of material); and fracture.
The presence, location and extent of each of these damage or failure modes is dependent on material properties, as well as on design, kinematics, and individual patient factors.
Total Hip Replacements.
In total hip replacements, the main mode of polyethylene failure is abrasion and burnishing which generate small ( less than 1xcexc diameter) polyethylene debris particles. These small particles caused by wear elicit a complex biological response which eventually leads to bone resorption which, in turn, causes implant loosening. On loosening of the implant, pain ensues and revision surgery becomes necessary. The process of bone loss due to particulate debris is termed osteolysis and is a major cause of hip replacement failure. Fractures of the polyethylene acetabular component in hip replacement devices are less common but do occur. Fracture of acetabular components has been shown to be design dependent.
Total Knee Replacements.
In total knee replacements, in addition to the generation of small debris particles, the damage to the polyethylene is often macroscopic in nature. Pitting, delamination and fracture are prevalent in total knee replacements. There have also been several reports which strongly associate pitting, delamination and fracture with decreases in polyethylene mechanical properties, specifically fracture and fatigue properties.
Previous Methods to Reduce Wear.
In the mid 1970""s, Oonishi in Japan reported improvements on the wear properties of polyethylene by using high dosages of gamma irradiation. He used 100 Mrads of gamma irradiation to treat acetabular cups made from high density polyethylene for implantation. This is far above the 2.5-4 Mrads generally used to sterilize these components. This high dose irradiation causes a high degree of cross linking in the polyethylene. This cross linking has-been shown to alter many of the properties of the polyethylene, including improving the wear properties. Oonishi demonstrated a factor reduction in wear.
In the last few years, increasing cross linking levels have been revisited for the purpose of improving wear properties. There have been several reports on crosslinking achieved by either gamma irradiation or electron beam irradiation. While crosslinking has produced improvements in the wear resistance of polyethylene, it has also been shown to reduce other mechanical properties, most notably, the fracture toughness of the polyethylene. Fracture toughness of a polymer is a measure of the tendency of the material to resist fracture. A reduction in fracture toughness is undesirable because it increases the risk of fracture in acetabular cups, a catastrophic failure necessitating immediate hip surgery. Further, it is feared that decreased polyethylene toughness will significantly increase the incidence of pitting, delamination and fracture in total knee replacements or indeed in any joint replacement device where the load exceeds the yield strength of the polyethylene. Accordingly, the amount of radiation that polyethylene materials received was 4 Mrads.
Sun et al., U.S. Pat. No. 5,728,748, describes oxidation-resistant medical implants prepared from olefinic materials having a molecular weight greater than 400,000. The irradiated material is heated in an oxygen deficient atmosphere to inactivate residual free radicals.
Saum et al., U.S. Pat. No. 6,017,975, describes a process for preparing a medical implant from ultrahigh molecular weight polyethylene which is irradiated and subsequently annealed at a temperature above 150xc2x0 C.
Accordingly, there is a need for polyethylene materials having both improved wear properties and not inferior fracture resistance. There is also a need for total joint replacement devices having improved wear properties such as those provided by cross linked polyethylene, without (or with less) attendant reduction in fracture toughness.
In one aspect, the invention is directed to a tough, wear resistant Ultra High Molecular weight Polyethylene shaped material prepared by the irradiation cross linking (using an irradiation dose higher than 4 Mrads, preferably 5 Mrads, and most preferably less than 10 Mrads of a UHMWPE article which has been shaped by direct compression molding. The product can used in total joint replacement devices such as hip, knee, elbow and shoulder.
In another aspect, the invention is directed to a total joint replacement device or component thereof comprising a shaped crosslinked article made from UHMWPE subjected to a process comprising direct compression molding followed with irradiation at a dose higher than 4 Mrads, preferably 5 Mrads, and most preferably less than 10 Mrads.